What is the thinnest film of a coating with n = 1.39 on glass (n=1.52) for which destructive interference of the red component (650 nm) of an incident white light beam in air can take place by reflection?

In optics, you have to create a layer of coating that is approximately 1/4 of the light's wavelength. The working equation for this problem is:

d = λ₀/4n,

where

λ₀ is the wavelength of the incident light

n is the refractive index of the coating

Substituting the values,

d = (650 nm) / 4 (1.39)

d = 116.9 nm